1. Technical Field
The present invention relates to a near field communication (NFC) transmitter and an NFC transmission method and more particularly, to an NFC transmitter using a delay-locked loop (DLL) and an NFC transmission method.
2. Related Art
An RFID technique is a kind of near field communication system and is a technique of acquiring information by exchanging data between an IC chip of an object and a terminal through electric waves (RF).
RFID tags can be classified into a type including only a memory device and a type including a CPU device having arithmetic operation capability and are generally widely used for a transportation card, an ID card, an electronic payment card, a distribution management card, and the like.
RFID tags are used to record information on an IC chip built therein or to read information recorded thereon and can be classified into an active or inductively coupled type and a passive or electromagnetic wave type depending on operating power supply systems.
The passive type receives and processes RF waves using a coil antenna and uses the processed RF waves as an operating power source, and has a demerit that the communicable range is short. The active type includes a power source such as a battery therein and the communicable range thereof may be several tens of meters.
As representative solutions in which the RFID technique is applied to a mobile communication terminal, the NFC standard ISO 18092, 14443 (A or B), and the like are standardized and an NFC chip is mounted on a mobile phone and the like. The NFC is a near-field bidirectional data communication technique using 13.56 MHz band which is an HF band and having a communicable range of about 5 cm 10 cm.
The NFC works a card emulation mode in which a mobile phone works like an existing RFID tag and information of a tag built in the mobile phone is recognized when the mobile phone approaches a reader, a reader/writer mode in which the mobile phone serves as a reader or writes personal information or the like to the mobile phone, and a P2P (Peer to Peer) mode in which plural devices communicate with each other.
The NFC can use the existing RFID infra and thus is considered as a communication system which can be easily compatible with the RFID and which has a wide application field.
The NFC is a near field communication system using the HF band and uses an amplitude shift keying (ASK) as a data modulation system. The ASK can be classified depending on the communication speed as described in Table 1.
TABLE 1ASK Depth depending on Communication SpeedCommunication speed106 Kbps212 Kbps424 Kbps14443-A100%8%~30%14443-B8%~12%15693100%NFC-IP100%8%~30%
FIGS. 1 and 2 are circuit diagrams illustrating structures of NFC transmitters according to the related art. FIGS. 1 and 2 show systems that control VDD to implement an ASK modulation.
In the structure of the transmitter shown in FIG. 1, only 100% ASK and 10% ASK can be designated as modulation depths. When the transmitter operates at the ASK depth of 100%, M1 and M3 output VDD to GND like an inverter. When the transmitter operates at the ASK depth of 10%, M2 is turned on and M1 and M3 output VDD to the source voltage of M2. The transmitter structure shown in FIG. 1 is disclosed in Kyung-won Min, Suk-Byung Chai, and Shiho Kim, “An analog Front-End Circuit for ISO/IEC 14443-Compatible RFID interrogators”, ETRI Journal, Volume 27, Number 6, December, 2004.
In the structure of the transmitter shown in FIG. 2, a clock CLK and a clock CLK with a deformed duty ratio are input to the MUX. When the transmitter operates at the ASK depth of 100%, the clock CLK is used. However, when the transmitter operates at the ASK depth of 10%, the clock with a deformed duty ratio is used. The structure of the transmitter shown in FIG. 2 is disclosed in Jung-hyun Cho, Jikon Kim, Jae-Whan Kim, Kyunil Lee, Kwang-Duk Ahn, and Shiho Kim, “An NFC transceiver with RF-powered RFID transponder mode,” IEEE Asian Solid-State Circuits Conference, Jeju, Korea, Nov. 12-14, 2007.
However, the NFC transmitters according to the related art have a problem in that it is difficult to implement the ASK having various modulation depths.
Specifically, in the structure of the transmitter according to the related art shown in FIG. 1, current consumption due to a connection structure of VDD and a resistor always occurs to maintain the source voltage of M2 and a large capacitor should be connected to the outside to stabilize the source voltage. There is also a problem in that since the structure is implemented using a method of controlling the VDD voltage and thus the resistor is much influenced by a variation in PVT (Process, Supply Voltage, Temperature) due to external circumstances, it is difficult to expect a stable output.
In the transmitter structure according to the related art show in FIG. 2, the delay cell varies very sensitively to the variation in PVT.
Therefore, there is a need for an NFC transmitter in which it is easier to implement an ASK depth and current consumption is smaller and which is less sensitive to a variation in PVT, compared with the NFC transmitters according to the related art.